It is known that arterial compliance, which is to say the elastic behaviour of the artery, is considered as indispensable to good knowledge of the physiology, physiopathology and therapy of the arterial system. This compliance is a function of the arterial pressure and, in order to establish it, one thus needs to have the instantaneous relationship which exists between the pressure and the diameter at a given point of the artery.
Propositions for measuring the pressure-diameter relationship have already been advanced, for instance in the study presented on pages 789 to 793 of the review "Arch. Mal. Coeur Nr. 6, 1987, where the visco-elastic behaviour of the aorta on a conscious dog is analyzed. The viscoelastic response of the aorta to the administration of hormones is observed in the cited study by analyzing the aortic pressure-diameter relationship. This relationship is established by means of a pressure microsensor which may be calibrated in situ and introduced through the left humeral artery and placed in the light of the descending aorta and of two piezoelectric crystals of 4 mm diameter, diametrally attached in the envelope of the proximal descending aorta.
The means which have just been suggested have an invasive character, which is to say, they affect the integrity of the organs in which they intervene. On the contrary, the method of the present invention and the arrangement for carrying it out call for non-invasive sensors which remain placed at the surface of the artery to be analyzed with no penetration into the surrounding tissues.
Non-invasive sensors permitting the continuous measurement of blood pressure are known. In particular, one may mention the photoplethysmograph sold by the Ohmeda Company, 3030 Airco Drive, Madison, Wis., USA and bearing the registered trademark "finapres" (for finger arterial pressure). As indicated, the apparatus measures the blood pressure at the end of a finger according to the method described in the article "Effects of Peripheral Vasoconstriction on the Measurement of Blood Pressure in a Finger" in the review Cardiovascular Research, 1985, 19, 139-145.
Non-invasive sensors enabling the measurement of the arterial diameter are also known. In particular, there is the apparatus employed in the U.S. patent document U.S. Pat. No. 4,370,985 which permits the measurement of the arterial diameter by sending an ultrasonic wave onto the artery and measuring the echoes sent back by the walls thereof. This diameter measurement may be effected on surface arteries, for instance the humeral artery or the radial artery.
From the brief description of the presently known sensors which has just been given hereinabove, it is apparent that it is not possible to measure non-invasively the pressure in every artery other than that of the finger and the diameter of said artery at the same place in a manner such that the relationship or pressure-diameter curve shows systematic hysteresis. This is due to the fact that the wave propagation velocity being finite, the pressure variations measured downstream show a certain delay relative to the corresponding diameter. To be sure, this delay is greater when the distance which separates the two measurement sites increases. This measurement defect must be thus corrected in order that the mechanical properties of the artery calculated from the pressure-diameter relationship are not artificially distorted.